1. Field of the Invention
This invention relates to the recording of information on film and particularly to the production of negative or positive film transparencies useful in the production of proof copies or for imaging printing plates.
2. Description of the Prior Art
Recently, many systems for imaging printing plates with laser beams have been proposed. U.S. Pat. No. 3,964,389 to John O. H. Peterson, which is herein incorporated by reference, discloses a method of making an imaged printing plate using a transparent substrate coated with (a) particles which absorb laser energy, (b) a self-oxidizing binder, and (c) a cross-linking agent or a cross-linking agent in combination with a cross-linkable resin. The coated side of the substrate is placed in intimate contact with a lithographic printing surface and subjected to a laser beam directed through the transparent substrate which by writing in the configuration of an image selectively transfers portions of the coating to the lithographic printing surface. The transferred coating is then rendered durable by heating the binder or resin to cross link it.
The transferred coating leaves a clear area on the transparent substrate which corresponds to the image areas on the lithographic printing surface. The laser-imaged transparent film thus constitutes a negative transparency of the image produced by the laser beam on the plate. Such a negative transparency is useful as a mask in the production of proof copies or for imaging conventional photo lithographic printing plates.
The Peterson patent discloses the use of carbon black particles in the coating to absorb laser energy. The Peterson coating can be transferred to the lithographic printing surface satisfactorily with acceptable laser power requirements, but more coating residue may be left behind on the transparent film than desirable for forming good negative masters. Increasing the power of the laser or slowing down the writing speed leaves less residue behind, but this is undesirable for obvious reasons.